Battery assembly and method of manufacturing nonaqueous electrolyte secondary battery

ABSTRACT

A battery assembly disclosed herein is a battery assembly before being subjected to initial charge. In the battery assembly, a positive electrode has a positive electrode mixture layer that contains a positive electrode active material and NMP, and an oxalate complex compound and FSO3Li are contained in a nonaqueous electrolyte solution. In the battery assembly disclosed herein, a NMP content in the positive electrode mixture layer is 50 ppm to 1500 ppm, the DBP oil absorption of the positive electrode active material is 30 ml/100 g to 45 ml/100 g, and a FSO3Li content in the nonaqueous electrolyte solution is 0.1 wt % to 1.0 wt %. With this, it is possible to prevent a reduction in input-output characteristics caused by formation of a film derived from NMP on the surface of the positive electrode active material, and hence it is possible to prevent an increase in facility cost and a reduction in manufacturing efficiency caused by adjustment of the content of NMP.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority based on Japanese PatentApplication No. 2018-88128 filed on May 1, 2018, the entire contents ofwhich are incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a battery assembly before beingsubjected to initial charge, and a method of manufacturing a nonaqueouselectrolyte secondary battery by performing the initial charge on thebattery assembly.

2. Description of the Related Art

In recent years, a nonaqueous electrolyte secondary battery such as alithium ion secondary battery is preferably used as what is called aportable power source for a personal computer or a cellular phone, or apower source for driving a vehicle. Among such nonaqueous electrolytesecondary batteries, the importance of a lithium ion secondary batterythat is light and capable of obtaining high energy density isparticularly increased. The lithium ion secondary battery is used as ahigh output power source used in a vehicle such as an electric vehicleor a hybrid vehicle (e.g., a power source for driving a motor coupled toa driving wheel of a vehicle).

In general, such a nonaqueous electrolyte secondary battery ismanufactured by fabricating a battery assembly in which an electrodebody and a nonaqueous electrolyte solution are accommodated in a case,and performing initial charge on the battery assembly. The electrodebody includes a sheet-shaped positive electrode in which a positiveelectrode mixture layer is provided on the surface of a foil-likepositive electrode current collector, and a sheet-shaped negativeelectrode in which a negative electrode mixture layer is provided on thesurface of a foil-like negative electrode current collector.

In the manufacture of such a nonaqueous electrolyte secondary battery,there are cases where part of the nonaqueous electrolyte solution(hereinafter also simply referred to as an “electrolyte solution”)resolves in the initial charge, and a film called a solid electrolyteinterface (SEI) film is formed on the surface of a negative electrodeactive material. When such an SEI film is formed, the negative electrodeis stabilized, and hence the subsequent resolution of the electrolyteresolution is inhibited.

However, the resolution of the electrolyte solution in the initialcharge described above is an irreversible reaction, and hence theresolution thereof leads to a reduction in battery capacity. To copewith this, in recent years, there is proposed a technique in which anadditive (hereinafter referred to as a “film-forming agent”) thatresolves at a potential lower than the resolution potential of theelectrolyte solution and forms the SEI film is added to the electrolytesolution in advance and a film derived from the film-forming agent isthereby formed.

An example of the technique for forming the film derived from such afilm-forming agent includes a technique described in Japanese PatentApplication Publication No. 2016-126908. In the technique described inthe document, an oxalate complex compound serving as the film-formingagent is added to a nonaqueous electrolyte solution, andN-methyl-2-pyrrolidone (hereinafter also referred to as “NMP”) is addedto a positive electrode active material layer (positive electrodemixture layer). With this, it is possible to form a film (SEI film) thatcontains an ingredient derived from NMP and an ingredient derived fromthe oxalate complex compound on the surface of an active material(typically, a negative electrode active material). By forming the SEIfilm derived from NMP and the oxalate complex compound, it is possibleto obtain a battery that is excellent in durability (e.g., hightemperature retention characteristics) as compared with the case wherethe SEI film derived only from the oxalate complex compound is formed.

SUMMARY

Incidentally, in one of means for forming the positive electrode mixturelayer containing NMP such as the technique described in Japanese PatentApplication Publication No. 2016-126908, NMP is used as a dispersionmedium that is used when a precursor of the positive electrode mixturelayer (positive electrode mixture paste) is prepared. However, in themeans, when a NMP content in the positive electrode mixture layer(residual NMP amount) is excessively high, there is a possibility that ahigh-resistance film will be formed on the surface of a positiveelectrode active material, and input-output characteristics will bereduced. To cope with this, in the conventional technique, the residualNMP amount in the positive electrode mixture layer is adjusted byperforming a heating and drying process on the positive electrodemixture paste applied to the surface of the positive electrode currentcollector and removing part of NMP.

However, in the conventional technique described above, in order toreliably adjust the residual NMP amount in the positive electrodemixture layer to an amount that does not cause a reduction ininput-output characteristics, it is necessary to perform the heating anddrying process for a long time. Accordingly, problems such as anincrease in facility cost caused by extension of a heating and dryingline at a manufacturing site, and a reduction in manufacturingefficiency caused by prolonged process time have occurred.

The present disclosure has been made in order to cope with suchproblems, and an object thereof is to provide a technique forefficiently manufacturing a nonaqueous electrolyte secondary battery inwhich a film containing an ingredient derived from NMP is formed on thesurface of a negative electrode active material at low cost.

In order to achieve the above object, a battery assembly having thefollowing configuration is provided as an aspect of the presentdisclosure.

Note that, as described above, the nonaqueous electrolyte secondarybattery is manufactured by performing initial charge (conditioningprocess) on a structure in which an electrode body and a nonaqueouselectrolyte solution are accommodated in a case. The “battery assembly”in the present specification denotes the structure before beingsubjected to the initial charge.

A battery assembly disclosed herein includes an electrode body having apositive electrode and a negative electrode, a nonaqueous electrolytesolution containing a nonaqueous solvent and a supporting electrolyte,and a case accommodating the electrode body and the nonaqueouselectrolyte solution. In the battery assembly, the positive electrodehas a positive electrode mixture layer that contains a granular positiveelectrode active material and N-methyl-2-pyrrolidone, and an oxalatecomplex compound and lithium fluorosulfonate (FSO₃Li) are contained inthe nonaqueous electrolyte solution.

In addition, in the battery assembly disclosed herein, the content ofN-methyl-2-pyrrolidone per unit mass of the positive electrode mixturelayer is 50 ppm to 1500 ppm, the DBP oil absorption of the positiveelectrode active material is 30 ml/100 g to 45 ml/100 g, and the contentof lithium fluorosulfonate is 0.1 wt % to 1.0 wt % when the total massof the nonaqueous electrolyte solution is 100 wt %.

Note that the “DBP oil absorption” in the present specification denotesa value measured based on JIS K6217-4 (2008) with dibutyl phthalate(DBP) used as a reagent liquid. In addition, the “content of NMP” in thepresent specification denotes the amount of NMP remaining in thepositive electrode mixture layer of the battery assembly beforesubjected to the initial charge (residual NMP amount), i.e., the NMPcontent after being adjusted by a heating and drying process or the likeunless otherwise explicitly specified.

First, in the battery assembly disclosed herein, FSO₃Li is contained inthe nonaqueous electrolyte solution. The FSO₃Li has the function ofresolving in the initial charge and causing an ingredient derived fromthe FSO₃Li to be absorbed on the surface of the positive electrodeactive material and form a film.

The present inventors have focused attention on the fact that theresolution and absorption of the FSO₃Li take place before the resolutionof NMP, and the fact that the resistance of a film derived from FSO₃Liis lower than that of a film derived from NMP. That is, the presentinventors have concluded that the low-resistance film derived fromFSO₃Li is formed on the surface of the positive electrode activematerial before the high-resistance film derived from NMP is formed onthe surface of the positive electrode active material by using thenonaqueous electrolyte solution containing FSO₃Li, and hence it ispossible to prevent a reduction in output characteristic caused by theformation of the film derived from NMP on the surface of the positiveelectrode active material.

In general, it is considered that, when the DBP oil absorption of thepositive electrode active material is increased, the number of reactionfields in charge and discharge is increased, and hence input-outputcharacteristics of a battery are improved. However, when the DBP oilabsorption of the positive electrode active material is excessively highin the case where the film derived from FSO₃Li is formed on the surfaceof the positive electrode active material, the number of reaction fieldsin charge and discharge is excessively increased, and there is apossibility that the formation of the high-resistance film derived fromNMP will start before the film derived from FSO₃Li properly covers thereaction fields. The present inventors have concluded that, inconsideration of the above possibility, it is necessary to adjust theDBP oil absorption of the positive electrode active material to apredetermined range in order to properly form the film derived fromFSO₃Li and obtain preferable input-output characteristics.

The battery assembly disclosed herein has been obtained by performingvarious tests by the present inventors based on the above knowledge.That is, in the battery assembly disclosed herein, the DBP oilabsorption of the positive electrode active material is adjusted to 30ml/100 g to 45 ml/100 g, and the content of FSO₃Li in the nonaqueouselectrolyte solution is adjusted to 0.1 wt % to 1.0 wt %. With this, itis possible to preferentially form the low-resistance film derived fromFSO₃Li on the surface of the positive electrode active material, andsuitably prevent a large amount of the high-resistance film derived fromNMP from being formed on the surface of the positive electrode activematerial.

Consequently, according to the battery assembly disclosed herein, evenwhen the residual NMP amount in the positive electrode mixture layer isincreased, it is possible to prevent a reduction in input-outputcharacteristics caused by the formation of the film derived from NMP (inother words, it is possible to increase the permissible amount of theresidual NMP amount to an amount larger than the conventionalpermissible amount thereof). Specifically, according to the batteryassembly disclosed herein, it is possible to increase the permissibleamount of the residual NMP to 1500 ppm. As a result, it is possible toreduce time required for the heating and drying process required toremove NMP, and hence it is possible to significantly contribute to areduction in facility cost and an improvement in manufacturingefficiency in the manufacture of the nonaqueous electrolyte secondarybattery.

Note that, in the battery assembly disclosed herein, at least a specificamount of NMP needs to be contained in the positive electrode mixturelayer for the initial purpose of improving the durability of thebattery. In order to enable the improvement in the durability, it isrequired that the content of NMP in the positive electrode mixture layeris 50 ppm or more.

In a preferred aspect of the battery assembly disclosed herein, thecontent of the oxalate complex compound is 0.05 wt % to 1.0 wt % whenthe total mass of the nonaqueous electrolyte solution is 100 wt %.

With this, when the initial charge is performed on the battery assembly,it is possible to suitably form an SEI film on the surface of an activematerial (typically, a negative electrode active material), and hence itis possible to suitably prevent a reduction in battery capacity causedby the resolution of the electrolyte solution.

In addition, as another aspect of the present disclosure, a method ofmanufacturing a nonaqueous electrolyte secondary battery having thefollowing configuration is provided.

The method of manufacturing the nonaqueous electrolyte secondary batterydisclosed herein is the method of manufacturing the nonaqueouselectrolyte secondary battery including an electrode body having apositive electrode and a negative electrode, a nonaqueous electrolytesolution containing a nonaqueous solvent and a supporting electrolyte,and a case accommodating the electrode body and the nonaqueouselectrolyte solution.

The manufacturing method includes the steps of: preparing the positiveelectrode having a positive electrode mixture layer that contains agranular positive electrode active material and N-methyl-2-pyrrolidone;preparing the nonaqueous electrolyte solution that contains an oxalatecomplex compound and lithium fluorosulfonate (FSO₃Li); accommodating theelectrode body and the nonaqueous electrolyte solution in the case tofabricate a battery assembly; and manufacturing the nonaqueouselectrolyte secondary battery by performing initial charge on thebattery assembly.

In addition, in the manufacturing method disclosed herein, the contentof N-methyl-2-pyrrolidone per unit mass of the positive electrodemixture layer is 50 ppm to 1500 ppm, the DBP oil absorption of thepositive electrode active material is 30 ml/100 g to 45 ml/100 g, andthe content of lithium fluorosulfonate is 0.1 wt % to 1.0 wt % when thetotal mass of the nonaqueous electrolyte solution is 100 wt %.

The manufacturing method disclosed herein manufactures the nonaqueouselectrolyte secondary battery by performing the initial charge on thebattery assembly of the above aspect. As described above, in the batteryassembly of the above aspect, FSO₃Li is contained in the nonaqueouselectrolyte solution, and the content of FSO₃Li in the nonaqueouselectrolyte solution and the DBP oil absorption of the positiveelectrode active material are set so as to fall within predeterminedranges. Consequently, by performing the initial charge on the batteryassembly, it is possible to preferentially form the low-resistance filmderived from FSO₃Li on the surface of the positive electrode activematerial, and prevent the formation of the high-resistance film derivedfrom NMP on the surface of the positive electrode active material. Withthis, it is possible to prevent a reduction in input-outputcharacteristics caused by the formation of the film derived from NMP,and hence it is possible to increase the permissible amount of theresidual NMP amount in the positive electrode mixture layer to an amountlarger than the conventional permissible amount thereof. Consequently,according to the manufacturing method disclosed herein, it is possibleto reduce time required for the heating and drying process for theadjustment of the NMP content, and significantly contribute to areduction in facility cost and an improvement in manufacturingefficiency in the manufacture of the nonaqueous electrolyte secondarybattery.

In addition, in a preferred aspect of the method of manufacturing thenonaqueous electrolyte secondary battery disclosed herein, the contentof the oxalate complex compound is 0.05 wt % to 1.0 wt % when the totalmass of the nonaqueous electrolyte solution is 100 wt %.

With this, it is possible to suitably form the SEI film on the surfaceof the active material (typically, the negative electrode activematerial), and hence it is possible to suitably prevent a reduction inbattery capacity caused by the resolution of the nonaqueous electrolytesolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a battery assemblyaccording to an embodiment of the present disclosure;

FIG. 2 is a perspective view schematically showing an electrode body inthe embodiment of the present disclosure; and

FIG. 3 is a schematic view for explaining positive and negativeelectrodes of a lithium ion secondary battery according to theembodiment of the present disclosure.

DETAILED DESCRIPTION

Hereinbelow, embodiments of the present disclosure will be described. Inthe drawings used in the following description, members and portionsthat have the same functions are designated by the same referencenumerals. Note that the dimensional relationship (length, width,thickness, and the like) in the individual drawings may not necessarilyreflect the actual dimensional relationship in an accurate manner. Apartfrom matters that are specifically mentioned in the presentspecification, other matters that are necessary for implementing thepresent disclosure (e.g., structures of a case and an electrode terminaland the like) can be understood as design matters of those skilled inthe art based on conventional techniques in the field.

Note that, in the case where a numerical range is indicated by “A to B”in the present specification, “A to B” means “not less than A and notmore than B”.

1. Battery Assembly

Hereinbelow, as an embodiment of the present disclosure, a lithium ionsecondary battery before being subjected to initial charge (conditioningprocess), i.e., a battery assembly of a lithium ion secondary batterywill be described. FIG. 1 is a perspective view schematically showingthe battery assembly according to the present embodiment, and FIG. 2 isa perspective view schematically showing an electrode body in thepresent embodiment.

(1) Case

As shown in FIG. 1, a battery assembly 1 according to the presentembodiment includes a flat square case 50. The case 50 is constituted bya flat case body 52 of which the upper surface is open, and a lid 54that covers an opening portion in the upper surface. The lid 54 servingas the upper surface of the case 50 is provided with a positiveelectrode terminal 70, a negative electrode terminal 72, and a liquidinlet 56.

(2) Electrode Body

In the battery assembly 1 according to the present embodiment, anelectrode body 80 shown in FIG. 2 is accommodated inside the case 50.The electrode body 80 includes a positive electrode 10, a negativeelectrode 20, and a separator 40. The positive electrode 10 and thenegative electrode 20 face each other via the separator 40.Specifically, the electrode body 80 is a wound electrode body formed bystacking the positive electrode 10 and the negative electrode 20 inlayers via the separator 40 and winding the multilayer body.

Hereinbelow, members constituting the electrode body 80 in the presentembodiment will be specifically described.

(a) Positive Electrode

As shown in FIG. 2, the positive electrode 10 is formed by providing apositive electrode mixture layer 14 on the surface (typically, bothsurfaces) of a positive electrode current collector 12. In addition, atone side edge portion of the positive electrode 10, a current collectorexposed portion 16 on which the positive electrode mixture layer 14 isnot provided is formed. Further, at one side edge portion of theelectrode body 80, a positive electrode connection portion 80 a aroundwhich the current collector exposed portion 16 is wound is formed, andthe positive electrode terminal 70 (see FIG. 1) is connected to thepositive electrode connection portion 80 a. Note that aluminum foil orthe like is used as the positive electrode current collector 12.

(a-1) Positive Electrode Active Material

The positive electrode mixture layer 14 contains a granular positiveelectrode active material. The positive electrode active materialincludes a lithium composite oxide that can occlude and release lithiumions. As such a positive electrode active material, a lithium compositeoxide containing one or more transition metal elements(lithium-transition metal composite oxide) is used. Examples of thelithium-transition metal composite oxide include a lithium-nickelcomposite oxide, a lithium-nickel-cobalt composite oxide, and alithium-nickel-cobalt-manganese composite oxide, and alithium-nickel-cobalt-manganese composite oxide having a layered rocksalt structure is typically used. Note that the type of the lithiumcomposite oxide that can be used as the positive electrode activematerial in the battery assembly disclosed herein is not particularlylimited, and hence the detailed description thereof will be omitted.

In the battery assembly 1 according to the present embodiment, the DBPoil absorption of the positive electrode active material described aboveis adjusted to a range of 30 ml/100 g to 45 ml/100 g. The range of theDBP oil absorption of the positive electrode active material may be setto 32.5 ml/100 g to 42.5 ml/100 g, and the example of DBP oil absorptionthereof is about 40 ml/100 g. Although described later in detail, byadjusting the DBP oil absorption of the positive electrode activematerial to 30 ml/100 g or more, it is possible to ensure sufficientreaction fields in charge and discharge, and hence it is possible tosuitably improve input-output characteristics. On the other hand, whenthe oil absorption of the positive electrode active material exceeds 45ml/100 g, the possibility that a film derived from NMP will be formed onthe surface of the positive electrode active material and input-outputcharacteristics will be reduced is increased. The positive electrodeactive material having the above DBP oil absorption can be obtained byusing a hollow lithium composite oxide having an internal cavity.

(a-2) N-Methyl-2-Pyrrolidone

Further, in the battery assembly 1 according to the present embodiment,the positive electrode mixture layer 14 contains N-methyl-2-pyrrolidone(NMP). In the battery assembly 1 according to the present embodiment,the NMP content per unit mass of the positive electrode mixture layer 14is adjusted to a predetermined range. Although described laterspecifically, according to the present embodiment, it is possible toprevent the formation of the film derived from NMP on the surface of thepositive electrode active material, and hence it is possible to increasethe permissible amount of residual NMP to 1500 ppm. On the other hand,from the viewpoint of improving the durability of the battery by formingan SEI film containing an ingredient derived from NMP on the surface ofa negative electrode active material, it is necessary to adjust thecontent of NMP in the positive electrode mixture layer to 50 ppm ormore. Consequently, in the battery assembly 1 according to the presentembodiment, the NMP content per unit mass of the positive electrodemixture layer 14 is adjusted to 50 ppm to 1500 ppm. The range of the NMPcontent may be set to 500 ppm to 1000 ppm, and the example of NMPcontent is 750 ppm.

(a-3) Other Additives

Note that the positive electrode mixture layer 14 may contain anadditive other than the above-described positive electrode activematerial. Examples of the additive include a conductive material and abinder. As the conductive material, it is possible to suitably use,e.g., carbon black such as acetylene black (AB) or a carbon materialsuch as graphite. In addition, as the binder, it is possible to use,e.g., polyvinylidene fluoride (PVdF), polyvinylidene chloride (PVdC), orpolyethylene oxide (PEO).

(b) Negative Electrode

As shown in FIG. 2, the negative electrode 20 is formed by providing anegative electrode mixture layer 24 on the surface (e.g., both surfaces)of a negative electrode current collector 22. At one side edge portionof the negative electrode 20, a current collector exposed portion 26 onwhich the negative electrode mixture layer 24 is not provided is formed.In addition, a negative electrode connection portion 80 b around whichthe current collector exposed portion 26 is wound is formed at one sideedge portion of the electrode body 80, and the negative electrodeterminal 72 (see FIG. 1) is connected to the negative electrodeconnection portion 80 b. Note that copper foil or the like is used asthe negative electrode current collector 22.

(b-1) Negative Electrode Active Material

The negative electrode mixture layer 24 contains a granular negativeelectrode active material. The negative electrode active materialincludes a carbon material that can occlude and release lithium ions. Asthe carbon material used as such a negative electrode active material,it is possible to use one or two or more materials that areconventionally used in the lithium ion secondary battery withoutparticular limitation. As the carbon material, for example, graphitecarbon (graphite), amorphous carbon, or amorphous coated graphite isused. Note that the type of the carbon material that can be used as thenegative electrode active material in the battery assembly disclosedherein is not particularly limited, and hence the detailed descriptionthereof will be omitted.

(b-2) Other Additives

The negative electrode mixture layer 24 may contain an additive otherthan the negative electrode active material. Examples of the additiveinclude a binder and a thickening agent. As the binder, it is possibleto use, e.g., polyvinylidene fluoride (PVDF) or styrene butadiene rubber(SBR) and, as the thickening agent, it is possible to use, e.g.,carboxymethyl cellulose (CMC) or the like.

(c) Separator

The separator 40 is disposed between the positive electrode 10 and thenegative electrode 20. The separator 40 is a porous insulating sheet inwhich a plurality of minute pores that allow passage of charge carriers(lithium ions) are formed. The diameter of the pore of the separator 40is about 0.01 μm to 6 μm. In the separator 40, it is possible to useinsulating resins such as, e.g., polyethylene (PE), polypropylene (PP),polyester, and polyamide. Note that the separator 40 may be a multilayersheet in which two or more layers of the above resin are stacked. Thethickness of the separator 40 may be set to 5 μm to 40 μm, 10 μm to 30μm, or even 15 μm to 25 μm. On the surface of the separator 40, a heatresistance layer (HRL layer) containing a metal oxide such as alumina(Al₂O₃) may be formed.

(3) Nonaqueous Electrolyte Solution

Although not shown in the drawings, in the battery assembly 1 accordingto the present embodiment, a nonaqueous electrolyte solution in which asupporting electrolyte and a film-forming agent are contained in anorganic solvent (nonaqueous solvent) is accommodated inside the case 50(see FIG. 1). Hereinbelow, the composition of the nonaqueous electrolytesolution in the present embodiment will be described.

(a) Nonaqueous Solvent

As the nonaqueous solvent, it is possible to use, e.g., various organicsolvents that are used in the nonaqueous electrolyte solution of acommon lithium ion secondary battery without particular limitation.Examples of the organic solvent include saturated cyclic carbonate,chain carbonate, chain carboxylate ester, cyclic carboxylate ester, anether-based compound, and a sulfone-based compound. In addition, theorganic solvents can be used alone or in combination of two or morethereof.

Note that, among the nonaqueous solvents, specific examples of thesaturated cyclic carbonate include ethylene carbonate, propylenecarbonate, and butylene carbonate. Specific examples of the chaincarbonate include dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, and di-n-propyl carbonate. Specific examples of the chaincarboxylate ester include methyl acetate, ethyl acetate, n-propylacetate, and n-butyl acetate. Specific examples of the cycliccarboxylate ester include gamma-butyrolactone, gamma-valerolactone,gamma-caprolactone, and epsilon-caprolactone. Specific examples of theether-based compound include diethyl ether, di (2-fluoroethyl) ether,and di (2, 2-difluoroethyl) ether. Specific examples of thesulfone-based compound include 2-methylsulfolane, 3-methylsulfolane,2-fluorosulfolane, and 3-fluoro sulfolane.

(b) Supporting Electrolyte

The supporting electrolyte is used as a main electrolyte, and a lithiumsalt such as, e.g., LiPF₆, LiBF₄, or LiClO₄, is suitably used. Thecontent of the supporting electrolyte is not particularly limited unlessthe effect of the present disclosure is significantly spoiled. Forexample, in the case where LiPF₆ is used as the supporting electrolyte,the mol concentration of LiPF₆ is adjusted to 0.5 mol/L to 3.0 mol/L.The range of the mol concentration of LiPF₆ may be set to 0.5 mol/L to1.5 mol/L, and the example of molarity thereof is 1 mol/L. It ispossible to achieve an appropriate balance between the total ion contentin the nonaqueous electrolyte solution and the viscosity of theelectrolyte solution by adjusting the content of LiPF₆ in the nonaqueouselectrolyte solution, and hence it is possible to improve input-outputcharacteristics without excessively reducing ion conductivity.

(c) Film-Forming Agent

As described above, in the battery assembly 1 according to the presentembodiment, the film-forming agent is contained in the nonaqueouselectrolyte solution. Specifically, as the film-forming agent, anoxalate complex compound and lithium fluorosulfonate are contained inthe nonaqueous electrolyte solution in the present embodiment.

(c-1) Oxalate Complex Compound

The oxalate complex compound is contained in the nonaqueous electrolytesolution in the present embodiment. An example of the oxalate complexcompound includes lithium bis (oxalate) borate (LiBOB). The oxalatecomplex compound is contained in the nonaqueous electrolyte solution,whereby, when initial charge is performed on the battery assembly 1, itis possible to form a negative electrode SEI film 29 (see FIG. 3)containing an ingredient derived from the oxalate complex compound onthe surface of a negative electrode active material 28, and prevent areduction in capacity caused by the resolution of the nonaqueouselectrolyte solution.

Note that the content of the oxalate complex compound when the totalmass of the nonaqueous electrolyte solution is 100 wt % may be adjustedto a range of 0.05 wt % to 1.0 wt %, 0.1 wt % to 0.75 wt %, and may beadjusted to, e.g., 0.5 wt %. With this, when the initial charge isperformed on the battery assembly, it is possible to suitably form thenegative electrode SEI film 29 on the surface of the negative electrodeactive material 28, and hence it is possible to suitably prevent areduction in battery capacity caused by the resolution of the nonaqueouselectrolyte solution.

(c-2) Lithium Fluorosulfonate

In addition, as described above, in the battery assembly 1 according tothe present embodiment, lithium fluorosulfonate (FSO₃Li) is contained inthe nonaqueous electrolyte solution. FSO₃Li has the function of beingabsorbed on the surface of the positive electrode active material andpreventing an increase in resistance, and has a characteristic that thespeed of resolution and absorption when the initial charge is performedis higher than that of the resolution of NMP. Consequently, as in thepresent embodiment, FSO₃Li is contained in the nonaqueous electrolytesolution, whereby it is possible to preferentially form a low-resistancefilm derived from FSO₃Li on the surface of the positive electrode activematerial before the high-resistance film derived from NMP is formed onthe surface of the positive electrode active material, and suitablyprevent a reduction in input-output characteristics caused by theformation of the film derived from NMP.

In addition, in the present embodiment, the content of FSO₃Li when thetotal mass of the nonaqueous electrolyte solution is 100 wt % is set to0.1 wt % or more. With this, it is possible to properly form the filmderived from FSO₃Li on the positive electrode active material, andsuitably prevent a reduction in input-output characteristics caused bythe formation of the film derived from NMP. Note that the content ofFSO₃Li may be set to 0.5 wt % or more.

On the other hand, when the content of FSO₃Li in the nonaqueouselectrolyte solution becomes excessively high, there is a possibilitythat the thickness of the film derived from FSO₃Li formed on the surfaceof the positive electrode active material will be excessively increased,and the excessive increase can cause a reduction in input-outputcharacteristics. In consideration of this, in the present embodiment,the content of FSO₃Li in the nonaqueous electrolyte solution is adjustedto 1.0 wt % or less. Note that the content of FSO₃Li may be set to 0.75wt % or less.

2. Method of Manufacturing Lithium Ion Secondary Battery

Next, as another embodiment of the present disclosure, a method ofmanufacturing the lithium ion secondary battery will be described.

The method of manufacturing the lithium ion secondary battery accordingto the present embodiment manufactures the lithium ion secondary batteryby performing the initial charge on the battery assembly 1 according tothe above-described embodiment. Specifically, the manufacturing methodaccording to the present embodiment includes the step of preparing thepositive electrode, the step of preparing the nonaqueous electrolytesolution, the step of fabricating the battery assembly, and the step ofperforming the initial charge on the battery assembly. Hereinafter, eachstep will be described.

(1) Positive Electrode Preparation Step

In the positive electrode preparation step, the positive electrodehaving the positive electrode mixture layer containing the granularpositive electrode active material and NMP is prepared. Specifically, inthe present step, the positive electrode active material having the DBPoil absorption of 30 ml/100 g to 45 ml/100 g is prepared first.Subsequently, a positive electrode mixture paste is prepared by mixingthe positive electrode active material and the additive (the conductivematerial or the binder) and kneading the mixture with a dispersionmedium. Then, after the positive electrode mixture paste is applied tothe surface of the positive electrode current collector, by performing aheating and drying process and a rolling process on the positiveelectrode mixture paste, the positive electrode in which the positiveelectrode mixture layer is provided on the surface of the positiveelectrode current collector is formed.

In the manufacturing method according to the present embodiment, in thepresent step, the positive electrode in which NMP having a content of 50ppm to 1500 ppm is contained in the positive electrode mixture layer isfabricated. In one of the means for forming the positive electrodemixture layer, NMP is used as the dispersion medium that is used whenthe positive electrode mixture paste is prepared, and conditions of theheating and drying process (temperature and time) are adjusted.

Note that, although a detailed description is omitted, similarly to thecommon method of manufacturing the lithium ion secondary battery, themethod of manufacturing the lithium ion secondary battery according tothe present embodiment includes the step of preparing the sheet-shapednegative electrode in which the negative electrode mixture layer isprovided on the surface of the negative electrode current collector, andthe step of fabricating the electrode body having the positiveelectrode, the negative electrode, and the separator. These steps can beperformed without particularly limiting procedures performed in theconventional method of manufacturing the lithium ion secondary battery,and hence the detailed description thereof will be omitted.

(2) Nonaqueous Electrolyte Solution Preparation Step

In the present step, the nonaqueous electrolyte solution containing theoxalate complex compound and FSO₃Li is prepared. Typically, thenonaqueous electrolyte solution is prepared by dissolving the supportingelectrolyte and the film-forming agent (the oxalate complex compound andFSO₃Li) in the above-described nonaqueous solvent. At this point, in themanufacturing method according to the present embodiment, the additionamounts of the individual materials are adjusted such that the contentof the oxalate complex compound is 0.05 wt % to 1.0 wt % and the contentof FSO₃Li is 0.1 wt % to 1.0 wt % relative to the total mass of thenonaqueous electrolyte solution (100 wt %).

(3) Battery Assembly Fabrication Step

In the present step, the battery assembly is fabricated by accommodatingthe electrode body and the nonaqueous electrolyte solution inside thecase. Specifically, first, the electrode body 80 (see FIG. 2) isaccommodated inside the case body 52 shown in FIG. 1. Subsequently, thepositive electrode terminal 70 provided in the lid 54 and the positiveelectrode connection portion 80 a (see FIG. 2) of the electrode body 80are electrically connected, and the negative electrode terminal 72 andthe negative electrode connection portion 80 b are electricallyconnected. Next, the case 50 is fabricated by covering the openingportion in the upper surface of the case body 52 with the lid 54 andwelding the case body 52 and the lid 54 together. Subsequently, thenonaqueous electrolyte solution is injected into the case 50 from theliquid inlet 56 provided in the lid 54, and the liquid inlet 56 is thensealed. With this, the battery assembly 1 in which the electrode body 80and the nonaqueous electrolyte solution are accommodated inside the case50 is fabricated.

(4) Initial Charge Step

In the manufacturing method according to the present embodiment, next,the nonaqueous electrolyte secondary battery is manufactured byperforming the initial charge (conditioning) on the fabricated batteryassembly 1. The conditions of the initial charge are not particularlylimited. The initial charge can be performed by performing constantcurrent-constant voltage charge (CC-CV charge) in which the batteryassembly 1 is charged until an inter-terminal voltage between thepositive electrode terminal 70 and the negative electrode terminal 72reaches 2.5 V to 4.2 V at a constant current of about 0.1 C to 10 C in aroom temperature environment (e.g., 25° C.), and is then charged untilstate of charge (SOC) reaches about 60% to 100% at a constant voltage.Note that the inter-terminal voltage in the initial charge may be set to3.0 V to 4.1 V. SOC may be set to about 80% to 100%.

FIG. 3 is a schematic view for explaining the states of the positive andnegative electrodes of the lithium ion secondary battery configured bythe initial charge.

As shown in FIG. 3, in the electrode body 80 in the present embodiment,the positive electrode 10 and the negative electrode 20 face each othervia the separator 40. As explained in the above embodiment, the positiveelectrode 10 includes the foil-like positive electrode current collector12, and the positive electrode mixture layer 14 containing the granularpositive electrode active material 18. In addition, the negativeelectrode 20 includes the foil-like negative electrode current collector22, and the negative electrode mixture layer 24 containing the granularnegative electrode active material 28.

In the present embodiment, when the initial charge is performed on thebattery assembly, the film (the negative electrode SEI film 29) isformed on the surface of the negative electrode active material 28, anda film (a positive electrode SEI film 19) is formed on the surface ofthe positive electrode active material 18.

Specifically, in the present embodiment, since the oxalate complexcompound (e.g., LiBOB) and NMP are contained in the nonaqueouselectrolyte solution, when the initial charge is performed on thebattery assembly, the oxalate complex compound and NMP resolve, and thenegative electrode SEI film 29 containing the ingredient derived fromthe oxalate complex compound and the ingredient derived from NMP isformed on the surface of the negative electrode active material 28.Thus, by forming the negative electrode SEI film 29 containing theingredient derived from the oxalate complex compound and the ingredientderived from NMP, it is possible to improve the durability of thebattery (e.g., high temperature retention performance).

Further, in the present embodiment, FSO₃Li is contained in thenonaqueous electrolyte solution. The FSO₃Li resolves before the NMPresolves when the initial charge is performed, and hence the ingredientderived from the FSO₃Li is absorbed on the surface of the positiveelectrode active material 18 before the ingredient derived from NMP.Consequently, in the present embodiment, the film (the positiveelectrode SEI film 19) derived from FSO₃Li is preferentially formed onthe surface of the positive electrode active material 18, and it ispossible to prevent the formation of the film derived from NMP on thesurface of the positive electrode active material 18. The resistance ofthe positive electrode SEI film 19 derived from the FSO₃Li is lower thanthat of the film derived from NMP, and hence it is possible to suitablyprevent a reduction in input-output characteristics caused by using thepositive electrode mixture layer 14 containing NMP.

In addition, in the present embodiment, the DBP oil absorption of thepositive electrode active material 18 is adjusted to a predeterminedrange such that the positive electrode SEI film 19 derived from theFSO₃Li is properly formed on the surface of the positive electrodeactive material 18. In general, when the DBP oil absorption of thepositive electrode active material is increased, the number of reactionfields in charge and discharge is increased, which is preferable for animprovement in input-output characteristics. However, as in the presentembodiment, in the case where the positive electrode SEI film derivedfrom FSO₃Li is formed, when the DBP oil absorption of the positiveelectrode active material is excessively increased, it takes apredetermined amount of time to form the film derived from FSO₃Li suchthat the film covers all of the reaction fields, and there is apossibility that the film derived from NMP will be formed on the surfaceof the positive electrode active material 18. Accordingly, in thepresent embodiment, the DBP oil absorption of the positive electrodeactive material 18 is adjusted to a range of 30 ml/100 g to 45 ml/100 g.

Thus, in the present embodiment, it is possible to preferentially formthe film derived from FSO₃Li on the surface of the positive electrodeactive material, and hence, in spite of the fact that the positiveelectrode containing NMP is used for improving durability, it ispossible to prevent the formation of the film derived from the NMP onthe surface of the positive electrode active material. Consequently,according to the present embodiment, even in the case where the residualNMP amount of about 1500 ppm is contained in the positive electrodemixture layer, it is possible to suitably prevent a reduction ininput-output characteristics (in other words, it is possible to increasethe permissible amount of the residual NMP amount to 1500 ppm). As aresult, it is possible to reduce time required for the heating anddrying process for removing NMP, and hence it is possible to contributeto a reduction in facility cost and an improvement in manufacturingefficiency in the manufacture of the nonaqueous electrolyte secondarybattery. Note that, from the viewpoint of improving the durability ofthe battery, it is required that a predetermined amount of NMP iscontained in the positive electrode mixture layer, and hence the contentof NMP in the positive electrode mixture layer is set to 50 ppm or morein the present embodiment.

3. Other Embodiments

Thus, the embodiments of the present disclosure have been described.However, the present disclosure is not limited to the above-describedembodiments, and may be appropriately changed on an as needed basis.

For example, while the above embodiment has described, as the means forforming the positive electrode mixture layer containing NMP, the meansthat uses NMP as the dispersion medium and adjusts the residual NMPamount using the heating and drying process by way of example, thepositive electrode preparation step of the present disclosure is notlimited to the means. For example, the positive electrode mixture layerthat doesn't contain NMP may be formed in advance, and NMP may be addedto the positive electrode mixture layer by using means such as spraying.According to the present disclosure, it is possible to increase thepermissible amount of the residual NMP amount in the positive electrodemixture layer, and hence, even when a large amount of NMP is added byspraying or the like, it is possible to prevent a reduction ininput-output characteristics caused by the formation of the film derivedfrom NMP. Consequently, it is possible to suitably prevent a reductionin manufacturing efficiency caused by manufacturing and abandoning thebattery whose input-output characteristics are significantly reduced.

In addition, the above embodiment describes the wound electrode bodyaround which the positive electrode and the negative electrode are woundvia the separator as an example of the electrode body. However, it isonly required that the electrode body used in the battery assemblydisclosed herein includes the positive electrode and the negativeelectrode, and the electrode body is not limited to the wound electrodebody. Another example of the electrode body includes a multilayerelectrode body in which a plurality of the positive electrodes and aplurality of the negative electrodes are stacked in layers via theseparators.

Test Example

Hereinbelow, a test example related to the present disclosure will bedescribed, and the description of the test example is not intended tolimit the present disclosure.

1. Samples 1 to 21

In the present test example, lithium ion secondary batteries of Samples1 to 21 were fabricated by fabricating 21 types of battery assembliesthat differed in the DBP oil absorption of the positive electrode activematerial (ml/100 g), the residual NMP amount (ppm) in the positiveelectrode mixture layer, and the content (wt %) of FSO₃Li in thenonaqueous electrolyte solution from each other, and performing theinitial charge on the battery assemblies. Hereinbelow, specificfabrication conditions will be described.

(1) Sample 1

In Sample 1, first, the positive electrode active material (thelithium/nickel/cobalt/manganese composite oxide having the layered rocksalt structure (Li_(1+x)Ni_(1/3)Co_(1/3)Mn_(1/3)O₂)) having the DBP oilabsorption based on JIS K6217-4 of 30 mg/100 g was prepared.Subsequently, the above positive electrode active material, theconductive material (acetylene black (AB)), and the binder(polyvinylidene fluoride (PVdF)) were mixed at a ratio of 90:8:2. Next,the positive electrode mixture paste was prepared by kneading themixture with N-methyl-2-pyrrolidone (NMP). Subsequently, thesheet-shaped positive electrode in which the positive electrode mixturelayers were provided on both surfaces of the positive electrode currentcollector was fabricated by applying the positive electrode mixturepaste to both surfaces of the belt-like positive electrode currentcollector (aluminum foil) and performing the heating and drying processon the positive electrode mixture paste, and then performing the rollingprocess thereon. Note that, in the present sample, an amount of time forthe heating and drying process was adjusted such that the NMP content(the residual NMP amount) in the positive electrode mixture layer was 50ppm.

Next, in the present test example, granular graphite was used as thenegative electrode active material. The negative electrode mixture pastewas prepared by mixing the negative electrode active material, thebinder (styrene butadiene rubber (SBR)), and the thickening agent(carboxymethyl cellulose (CMC)) at a ratio of 98:1:1, and then kneadingthe mixture with the dispersion medium (ion-exchanged water).Subsequently, the sheet-shaped negative electrode in which the negativeelectrode mixture layers were provided on both surfaces of the negativeelectrode current collector was fabricated by applying the negativeelectrode mixture paste to both surfaces of the negative electrodecurrent collector (copper foil) and performing the heating and dryingprocess on the negative electrode mixture paste, and then performing therolling process thereon.

Next, the flat wound electrode body was fabricated by stacking thepositive electrode and the negative electrode fabricated in the abovemanner in layers via the sheet-shaped separator and winding and pressingthe multilayer body. Subsequently, the fabricated wound electrode bodywas connected to the electrode terminals (the positive electrodeterminal and the negative electrode terminal) and then accommodatedinside the case body, and the case body and the lid were bondedtogether. Note that the separator used in the present test example is aseparator having a three-layer structure (PP/PE/PP) in which apolyethylene (PE) layer is sandwiched between two polypropylene (PP)layers.

Next, a liquid mixture was prepared. In the liquid mixture, thesupporting electrolyte (LiPF₆) was dissolved at a concentration of about1 mol/L in the nonaqueous solvent that contained ethylene carbonate(EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at avolume ratio of 3:4:3. Subsequently, the nonaqueous electrolyte solutionwas prepared by dissolving LiBOB and FSO₃Li in the liquid mixture suchthat the content of the oxalate complex compound (LiBOB) was 0.5 wt %,and the content of lithium fluorosulfonate (FSO₃Li) was 0.1 wt % whenthe prepared nonaqueous electrolyte solution was 100 wt %.

Next, the battery assembly was fabricated by injecting the abovenonaqueous electrolyte solution into the case from the liquid inlet andsealing the liquid inlet.

Subsequently, the lithium ion secondary battery for the test wasfabricated by performing the initial charge on the battery assembly.Specifically, first, the above battery assembly was disposed in anenvironment of 25° C. and was charged at a constant current of ⅓ C (CCcharge) until the voltage between the positive electrode terminal andthe negative electrode terminal reached 4.1 V, and the initial chargewas suspended for 10 minutes. Next, the battery assembly was dischargedat the constant current of ⅓ C (CC discharge) until the voltage betweenthe positive electrode terminal and the negative electrode terminalreached 3.0 V, and was discharged at a constant voltage (CV discharge)until the total discharge time reached 1.5 hours, and the initial chargewas suspended for 10 minutes. Subsequently, initial charge/discharge(conditioning) was performed by repeating the charge/discharge pattern,which corresponded to one cycle, three times (three cycles).

(2) Samples 2 to 21

In each of Samples 2 to 21, as shown in Table 1 described later, thelithium ion secondary battery for the test was fabricated according tothe same procedures as those of Sample 1 except that each of the DBP oilabsorption of the positive electrode active material, the residual NMPamount in the positive electrode mixture layer, and the content ofFSO₃Li in the nonaqueous electrolyte solution varied.

2. Evaluation Test

In the present evaluation test, output characteristics of the lithiumion secondary batteries of Samples 1 to 21 described above were measuredaccording to the following procedures. Note that the measurement resultis shown in Table 1.

Procedure 1: The battery is charged by constant current charge of 1 Cuntil SOC reaches 25% from 3.0 V in the room temperature environment(25° C.).

Procedure 2: The battery of which the SOC is adjusted to 25% is left tostand in a constant temperature bath of −30° C. for six hours.

Procedure 3: After Procedure 2, the battery is discharged at a constantwatt (W) from the SOC of 25% in a temperature environment of −30° C. Atthis point, the number of seconds from the start of the discharge untilthe voltage reaches 2.0 V is measured.

Procedure 4: Procedures 1 to 3 are repeated while the constant wattdischarge voltage of Procedure 3 is changed under conditions of 80 W to200 W. Herein, the constant watt discharge voltage of Procedure 3 isincreased so that the constant watt increases 10 W in each execution ofProcedures 1 to 3, that is, the constant watt discharge voltage ofProcedure 3 is such a voltage that the constant watt becomes 80 W in thefirst execution of Procedures 1 to 3, the voltage is such a voltage thatthe constant watt becomes 90 W in the second execution, 100 W in thethird execution, and so on. and Procedures 1 to 3 are repeated until theconstant watt discharge voltage of Procedure 3 reaches such a voltagethat the constant watt becomes 200 W.

Procedure 5: W at two seconds is calculated as the output characteristicfrom an approximate curve in a plot in which the horizontal axisindicates the number of seconds from the start of the discharge untilthe voltage reaches 2.0 V that is measured under each of the constantwatt conditions in Procedure 4 and the vertical axis indicates Wcorresponding to the number of seconds.

Procedure 6: The output characteristic of each sample in the case wherethe output characteristic of the lithium ion secondary battery of Sample20 is used as a reference (100) is calculated based on the outputcharacteristic obtained in Procedure 5.

TABLE 1 DBP oil residual NMP FSO₃Li absorption amount content outputSample (mg/100 g) (ppm) (wt %) characteristic 1 30 50 0.1 123 2 30 50 1127 3 30 1500 0.1 121 4 30 1500 1 126 5 45 50 0.1 132 6 45 50 1 134 7 451500 0.1 131 8 45 1500 1 132 9 30 750 0.1 122 10 30 1000 0.1 121 11 3040 0 88 12 30 50 0 85 13 30 1500 0 72 14 30 40 0.08 92 15 30 40 1.1 9516 30 2000 0.08 89 17 30 2000 1.1 98 18 26 50 0.1 99 19 26 2000 0.1 8620 47 50 1 100 21 47 2000 1 89

As shown in Table 1, very preferable output characteristics of 120 ormore were observed in the lithium ion secondary batteries of Samples 1to 10. From this, it was found that, in the case where the residual NMPamount in the positive electrode mixture layer was 50 ppm to 1500 ppm,it was possible to prevent a reduction in output characteristic causedby the residual NMP by adjusting the DBP oil absorption of the positiveelectrode active material to a range of 30 ml/100 g to 45 ml/100 g andadjusting the content of FSO₃Li in the nonaqueous electrolyte solutionto 0.1 wt % to 1 wt %. In other words, it was determined that it waspossible to increase the permissible amount of the residual NMP in thepositive electrode mixture layer to 1500 ppm by adjusting the DBP oilabsorption of the positive electrode active material to the range of 30ml/100 g to 45 ml/100 g and adjusting the content of FSO₃Li in thenonaqueous electrolyte solution to 0.1 wt % to 1 wt %.

Note that, in any of Samples 11 to 13, the output characteristic wassignificantly lower than those of Samples 1 to 4. Particularly in Sample11, in spite of the fact that the residual NMP amount was smaller thanthose of Samples 1 to 4, the output characteristic was lower than thoseof Samples 1 to 4. From this, it was determined that it was necessary toadd FSO₃Li to the nonaqueous electrolyte solution in order to prevent areduction in output characteristic caused by the residual NMP.

In Sample 14, in spite of the fact that the residual NMP amount wassmaller than that of Sample 1, the output characteristic was lower thanthat of Sample 1. This may be because the FSO₃Li content in thenonaqueous electrolyte solution was excessively low and the film derivedfrom the FSO₃Li was not adequately formed. From this, it was found thatit was necessary to adjust the content of FSO₃Li in the nonaqueouselectrolyte solution to 0.1 wt % or more in order to prevent a reductionin output characteristic caused by the residual NMP.

In Sample 15, in spite of the fact that the residual NMP amount wassmaller than that of Sample 2, the output characteristic was lower thanthat of Sample 2. This may be because the FSO₃Li content in thenonaqueous electrolyte solution was excessively high, the thickness ofthe film formed on the surface of the positive electrode active materialwas increased, and resistance in the positive electrode was increased.From this, it was found that, in the case where FSO₃Li was added to thenonaqueous electrolyte solution, it was necessary to adjust the contentof the FSO₃Li to 1 wt % or less.

In Sample 18, in spite of the fact that 0.1 wt % of FSO₃Li was containedin the nonaqueous electrolyte solution, the output characteristic waslower than that of Sample 1. This may be because the number of reactionfields in charge and discharge is reduced when the DBP oil absorption ofthe positive electrode active material is excessively reduced. Fromthis, it was found that it was necessary to adjust the DBP oilabsorption of the positive electrode active material to 30 mg/100 g ormore.

On the other hand, in Sample 20, in spite of the fact that 1 wt % ofFSO₃Li was contained in the nonaqueous electrolyte solution, the outputcharacteristic was lower than that of Sample 2. This may be because,when the DBP oil absorption of the positive electrode active materialwas excessively high, the number of reaction fields in charge anddischarge became excessively large, and the formation of the filmderived from NMP started before the reaction fields were covered withthe film derived from FSO₃Li. From this, it was found that, in the casewhere FSO₃Li was added to the nonaqueous electrolyte solution, it wasnecessary to adjust the DBP oil absorption of the positive electrodeactive material to 45 mg/100 g or less.

While the present disclosure has been described in detail, the aboveembodiments are only illustrative, and the disclosure disclosed hereinencompasses various changes and modifications to the specific examplesdescribed above.

What is claimed is:
 1. A battery assembly before being subjected toinitial charge, comprising: an electrode body having a positiveelectrode and a negative electrode; a nonaqueous electrolyte solutioncontaining a nonaqueous solvent and a supporting electrolyte; and a caseaccommodating the electrode body and the nonaqueous electrolytesolution, wherein the positive electrode has a positive electrodemixture layer that contains a granular positive electrode activematerial and N-methyl-2-pyrrolidone, an oxalate complex compound andlithium fluorosulfonate (FSO₃Li) are contained in the nonaqueouselectrolyte solution, a content of the N-methyl-2-pyrrolidone per unitmass of the positive electrode mixture layer is 50 ppm to 1500 ppm, aDBP oil absorption of the positive electrode active material is 30ml/100 g to 45 ml/100 g, and a content of the lithium fluorosulfonate is0.1 wt % to 1.0 wt % when a total mass of the nonaqueous electrolytesolution is 100 wt %.
 2. The battery assembly according to claim 1,wherein a content of the oxalate complex compound is 0.05 wt % to 1.0 wt% when the total mass of the nonaqueous electrolyte solution is 100 wt%.
 3. A method of manufacturing a nonaqueous electrolyte secondarybattery including an electrode body having a positive electrode and anegative electrode, a nonaqueous electrolyte solution containing anonaqueous solvent and a supporting electrolyte, and a caseaccommodating the electrode body and the nonaqueous electrolytesolution, the method comprising the steps of: preparing the positiveelectrode having a positive electrode mixture layer that contains agranular positive electrode active material and N-methyl-2-pyrrolidone;preparing the nonaqueous electrolyte solution that contains an oxalatecomplex compound and lithium fluorosulfonate (FSO₃Li); accommodating theelectrode body and the nonaqueous electrolyte solution in the case tofabricate a battery assembly; and manufacturing the nonaqueouselectrolyte secondary battery by performing initial charge on thebattery assembly, wherein a content of the N-methyl-2-pyrrolidone perunit mass of the positive electrode mixture layer is 50 ppm to 1500 ppm,a DBP oil absorption of the positive electrode active material is 30ml/100 g to 45 ml/100 g, and a content of the lithium fluorosulfonate is0.1 wt % to 1.0 wt % when a total mass of the nonaqueous electrolytesolution is 100 wt %.
 4. The method of manufacturing a nonaqueouselectrolyte secondary battery according to claim 3, wherein a content ofthe oxalate complex compound is 0.05 wt % to 1.0 wt % when the totalmass of the nonaqueous electrolyte solution is 100 wt %.